Can I use blender to create a model and 3D print it for some real device
Yes, you can do this.

To get realistic measurements, go to the Scene tab in the Properties panel and then find the Units section and select Metric.



Keep in mind, though, that once you export, all the coordinates are still in "Fundamental Blender Units." There's no information stored in the model about what measurement system was used to make the model. The target system (SketchFab or Shapeways or whoever) will just make an assumption about what units the coordinates are in (often centimeters). You'll have to do a little testing to synchronize Blender's unit system with your target's unit system. It should be a very straightforward conversion, though.

I'd recommend making a unit cube in Blender and exporting that to whatever your target system is. See how they interpret its size. You can then use the Scale setting in that same Units section to control what unit a Fundamental Blender Unit maps to. If your 1m^3 cube is interpreted to be 1cm^3 by the target system, then you just have to adjust your scale by 10^3 or 10^-3 (I'm not sure which).

The smallest length Blender can handle is 1μm. It can't go smaller than that but that should be enough for your use. You can simply type in units (cm, m, mm etc.) but for μm you have to use "um".

It would also be worth your time to take a look at  the addon measureit. It will make measuring things in Blender much easier.